1. Field of the Invention
The preset invention relates to an apparatus and a method for performing secret communication in order to avoid illegal eavesdropping and interception by a third party and, more particularly, relates to a data transmitting apparatus, a data receiving apparatus and a data transmitting method for performing data communication through selecting and setting a specific encoding/decoding (modulating/demodulating) method between a legitimate transmitter and a legitimate receiver.
2. Description of the Related Art
Conventionally, in order to perform secret communication between specific parties, there has been adopted a structure for realizing secret communication by sharing key information for encoding/decoding between transmitting and receiving ends and by performing, based on the key information, an operation/inverse operation on information data (plain text) to be transmitted, in a mathematical manner. FIG. 17 is a block diagram showing a structure of a conventional data communication apparatus based on the above-described structure.
In FIG. 17, the conventional data communication apparatus has a configuration in which a data transmitting apparatus 9001 and a data receiving apparatus 9002 are connected to each other via a transmission line 913. The data transmitting apparatus 9001 includes an encoding section 911 and a modulator section 912. The data receiving apparatus 9002 includes a demodulator section 914 and a decoding section 915.
In the data transmitting apparatus 9001, information data 90 and first key information 91 are inputted to the encoding section 911. The encoding section 911 encodes (modulates), based on the first key information 91, the information data 90. The modulator section 912 converts, in a predetermined demodulation method, the information data 90 encoded by the encoding section 911 into a modulated (modulating) signal 94 which is then transmitted to the transmission line 913.
In the data receiving apparatus 9002, the demodulator section 914 demodulates, in a predetermined demodulation method, the modulated (modulating) signal 94 transmitted via the transmission line 913. To the decoding section 915, second key information 96 which has the same content as the first key information 91 is inputted. The decoding section 915 demodulates (decrypts), based on the second key information 96, the modulated (modulating) signal 94 and then outputs information data 98.
Here, eavesdropping by a third party will be described by using an eavesdropper receiving apparatus 9003. In FIG. 17, eavesdropper receiving apparatus 9003 includes an eavesdropper demodulator section 916 and an eavesdropper decoding section 917.
The eavesdropper demodulator section 916 demodulates, in a predetermined demodulation method, the modulated (modulating) signal 94 transmitted via the transmission line 913. The eavesdropper decoding section 917 attempts, based on third key information 99, decoding of a signal demodulated by the eavesdropper demodulator section 916. Here, since the eavesdropper decoding section 917 attempts, based on the third key information 99 which is different in content from the first key information 91, decoding of the signal demodulated by the eavesdropper demodulator section 916, the information data 98 cannot be reproduced accurately.
A mathematical encryption (or also referred to as a computational encryption or a software encryption) technique based on such mathematical operation may be applicable to an access system described in Japanese Laid-Open Patent Publication No. 9-205420 (hereinafter referred to as Patent Document 1), for example. That is, in a PON (Passive Optical Network) system in which an optical signal transmitted from an optical transmitter is divided by an optical coupler and distributed to optical receivers at a plurality of optical subscribers' houses, such optical signals that are not desired and aimed at another subscribers are inputted to each of the optical receivers. Therefore, the PON system encrypts information data for each of the subscribers by using key information which is different by the subscribers, thereby preventing a leakage/eavesdropping of mutual information data and realizing safe data communication.
Further, the mathematical encryption technique is described in “Cryptography and Network Security: Principles and Practice” translated by Keiichiro Ishibashi et al., Pearson Education, 2001 (hereinafter referred to as Non-patent Document 1) and “Applied Cryptography” translated by Mayumi Adachi et al., Softbank publishing, 2003 (hereinafter referred to as Non-patent Document 2).
Among the mathematical encryption, a method called a stream encryption has a simple structure in which a cipher text is generated by performing an XOR operation between a pseudo-random number sequence outputted by a pseudo-random number generator and information data (a plain text) to be encrypted, and thus is advantageous for an increase in speed. On the other hand, the method is disadvantageous in that security in the stream encryption depends only on the pseudo-random number generator. That is, if the eavesdropper can obtain a combination of the plain text and the cipher text, the pseudo-random number series can be identified accurately (this is generally called a known-plain-text attack). Further, since an initial value of the pseudo-random number generator, that is, the key information and the pseudo-random number series correspond to each other uniquely, the key information can be figured out certainly if any decryption algorithm is applied. Further, a processing speed of a computer has been improved remarkably in recent years, and thus there has been a problem in that there is an increasing danger of decryption of the cipher text within a practical time period.